This invention relates to electrical resistance devices, and more particularly to devices having a resistance element which is trimmed to a predetermined resistance value, and to methods for making the same.
Resistance elements intended for use in a potentiometer or the like are frequently manufactured to a particular resistance value, within a given tolerance range, by depositing a resistive material having a resistance value less than the desired value onto a substrate. Portions of the resistive material are then removed from the resistance path until the desired value is achieved. Various devices, such as abrasive or grinding apparatus, are available for removing resistive material, although presently a laser is preferred where rapid trimming of a large volume of elements is required. A relatively simple trimming method entails the placement of sensors at opposite ends of the element to measure its total resistance, and then trimming until the measured resistance increases to the desired level. While adequate for applications in which the total resistance is of primary importance, this method does not provide sufficient linearity, i.e., a steady and regular increase in resistance as the element is traversed from one end to the other, required for certain applications such as high tolerance potentiometers.
A more sophisticated trimming method known in the prior art involves the placement of sensing probes at regular intervals along the length of the element, in effect dividing the element into a series of small resistance sections, and trimming to a desired resistance value for each section. Since each section is trimmed to a predetermined value within a given tolerance range, accurate placement of the sensing probes is obviously of great importance for this method. An early trimming technique employed probes which were physically moved from point to point along the element to sense the resistance of each section as it was trimmed. This technique, however, did not provide the consistent accuracy of probe placement required by high tolerance elements. A more recent approach, disclosed in U.S. Pat. No. 3,821,845 issued July 2, 1974, involves a contact point board of similar size and shape to the resistance element to be trimmed. Contacts are distributed along the board at intervals corresponding to the resistor sections and extend through the board from its upper to its under side. The board is placed over the element with the underside portion of each contact touching the element. The portions of the contacts on the upper side of the board are then connected to a laser control, enabling selected contacts to provide an indication of the section resistances as trimming proceeds. While this method can provide an accurate and repeatable placement of the contacts relative to each other, care must still be exercised to be sure that the board is placed in the same location over each element relative to the position of the elements themselves and their terminals. Also, the electrical connection between each contact and the element may be imperfect, thereby introducing a contact resistance that will throw off the accuracy of the readings obtained from the contacts. In addition, this method requires the separate manufacturing step of emplacing the contact board.